Which of these numbers is prime? ${3,\ 28,\ 39,\ 69,\ 81}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 3 are 1 and 3. The factors of 28 are 1, 2, 4, 7, 14, and 28. The factors of 39 are 1, 3, 13, and 39. The factors of 69 are 1, 3, 23, and 69. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 3 is a prime number.